Apparatus for radio communication.



G. W. PIOKARD.

APPARATUS FOR RADIO oommumcmon.

APPLICATION FILED APR.24,1913' mmgg Patented Feb. 9, 1915.

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903 7060504030. 113: K7|2345b m Inventor:

G. W. PIOKARD.

APPARATUS FOR RADIO UOMMUNICATION.

APPLICATION FILED APR.2-1,1913.

2 SHEETS-SHEET Z.

A;3"*"s In ventor:

awwmmaw Patented Feb. 9, 1915.

GREENLEAF WHITTIER PICKARD, OF AMESBURY. MASSACHUSETTS, ASSIGNOR TO WIRELESS SPECIALTY APPARATUS COMPANY,

A CORPORATIQN 03: NEW YORK.

01 BOSTON, MASSACHUSETTS,

APPARATUS FOR RADIO COMMUNICATION.

Specification of Letters Patent.

Patented Feb. 9, 1915.

Continuation of application Serial No. 461,617, filed November 9. 1908. This application filed April 24, 1913.

. Serial No. 763,408.

To all whom it may concern Be it known that I. GREENLEAF nrr'rnzu PICKARD, a citizen of the United States of America. and a resident of Amesbury, Massachusetts, have invented certain new and useful Improvements in Apparatus for Radio Communication, the principles of which are set forth in the following specification and accompanying drawing, which disclose the form of the invention which I now consider to be the best of various forms in which the principles of the invention may e embodied.

This application a continuation of that liled by me November 9. 1908, SerialNo. 461,617.

This invention relates to tuning inductances of the type wherein the turns of a winding are mechanically fixed, for use as in the high frequency oscillating-current circuits employed in w ir'eless telegraphy and telephony, or radio communication; and the invention has more particular relation to the receiving arrangements employedin radio communication wherein are employed -oscillation-current frequencies of the order of hundreds of thousands per second. Inductance tuners in this art of high frequency currents are, as well known to those skilled therein. of the well-known type in which the coil is wound continuously with turns close together, the successive turns being in mutually inductive relation, and without my coreof magnetic material, whereby the :auil is adapted as an inductance element of a resonant circuit of the high-frequency oscillating-current art, and whereby such inductance, when variable, is an adjustable inning-.8 element of such resonant oscillatory current circuit. That which I herein specifically term the mechanically fixed windin; type of high-frequency inductance- UHIQI'S has inherently certain important advantages over the movable winding type of the same general functioning, but wherein the same object of tuning, (i. 6., adjusting' the oscillation frequency for natural period of one or more of the resonant highfrequency circuits employed in this branch of the electrical art), is accomplished by unwinding a part of the wire from the tun:

ing coil so that the unwound portion is removed from the field of the coil itself, thereby decreasing the total inductance of the coil and thence varying the product of inductance and capacity in the high-frequency circuit to the extent desired to obtain the proper tune or natural period thereof. It is not here necessary to specify its points of inferiority to the fixed winding type, on which the present invention is an improvement.

The object of the inventionis a simple and etlicient device of the fixed winding type indicated generally in Figure 1,' and to take greater advantage than heretofore of its inherent superiority to the movable winding type, particularly in tuning the high-frequency receiving circuit or circuits of radio communication to electric waves received from a distant radiator or sender,

The fixed winding type has been embodied theoretically in several specific forms. but the one'which has been in general use prior to my invention is the movable contact-oncoil type lrnown long prior thereto in the high-frequency current art.

Broadly, the fixed winding type is divided into two classes, one wherein the inductance is cut in or out continuously or -gradually, without separate steps, and the other where it is cut in or out by successive steps of greater or less degree. The above contact-cn-coil device is an example of the step -by -step method. The continuous method is that where a conductor 01' contact was shifted along a wire in the resonant l'iigh-frequency circuit for the purpose, as its original users stated, of varying the coufiicient of self-induction of the wire and thereby adjusting the circuit to a definite period of oscillation-frequency or tune. This continuous-yariation method possesses the very important advantage of very fine ;regulation and close tuning, but is impracticable for commercial operation onaccount of the excessive range of contact movement required to. be made within a very limited time, and'on account of the bulky shape of the inductance,. both of which are necessarilyinvolved inan apparatus of this type. r r

Thecontact-on-coil tuner, of the second type, of'step-by-step variation, was used in general practice prior to my invention, in

preference to the'continuous-variation type tion type and the stepby-steptype, as used in the practice of radio telegraphyprior .to

this invcntiom'have. been alwaysinaform wherein the contact maker or switch operates directly on the inductance wire, this i being for a reason hereinafter explained in connection withcertain capacity phenomena. It has been proposed frequently to'providef the coil with taps to suitable switch-coir. Y tacts, but although'this expedient has been adopted in other branches oftheelectrical a1t,'its' use in high-frequency currentwork was prevented by certain difiidulties ,tosbe described herein which are overcome by the present invention, nder the more ractical 'stepby -stepwa, riationsub type ot the fixed windingtype, two distinct methods of control have been. suggested,' z'. 'e., one wherein the 'out-of-service. portions. of the coil are short-circuited, and another wherein the out-of-srvice .portions are simply dead-ended; but as the dif ference between these two is immaterial to the present invention, it 'is sufiicient simply to refer to the fact.

The specific form of the step-by-step type in most general prior use, has been thatof the contact-on-coil, su m. Inasmuch as that the type on whic the present invention -1s an improvement, the invention may be best understood by a brief consideration ofxthe advantages and disadvantages of the former, in connection with the drawings herein. A consideration of the-advantages of the contact-on-coil form willshow why it continued in commercial use for so many. years'and until displaced by the present invention and therefore Why, certain methods... employed in it are retained in. the present invention; and consideration, of the disadvantages of the contact-on-coil' system, in. connection with the invention disclosed herein will be useful in arrivingat an understandingof'the care required to be exer-:- cised by the constructor in embodying the, present invention in practical apparatus, and of the difficulties involved in theproblem of overcoming suchidi'sadvantages of the, prior devices.

Ofthe drawings, Fig.1 is an elevation of an inductance tuning .;apparatus; F ig. 2. is a diagrammatic -Vl WSilOWlllgflOIlVQDtlOnally; the high-frequency resonantcircuitA,

G to. be timed; euchwa iheapteana r erial,

contacts, Fig. 4 is an elevation of the box showing the outside thereof, presenting the reverse dr outside front of the box-side of Fig. 3; and Figs. 5 to 8 are like diagrammatic illustrations of various electrical forms in which the tuning inductance and switching arrangements may be, employed within the invention, h in connection -with the features more particularlyshown. in Fig, 3.

Fig. 1 herein indicates enerally the self'- inductance 'winding P of t e high-frequenc coiltypefi. .e., with-nonmagnetic corei" which may be employed. in, combination with V thearrangelnents shown in Fig. 3. In'Figl 1 the coil 'Pis shown as wound on..a sup-' porting core C, which as a .nonemagnetic core.,,nra'y"consist,;of ,,a cylindrical shell of. hard rubber. When-desired, the coil P ma v be used .with a second .coil X on a similar core .C'i in which case .coil P may be the primary and X'the'secondary of a high-tre-l quency I oscillation-transformn In either case the coil P may be-included in the usual antenna-to-ground"circuit A, G of radiov systems asshownin Fig. 2; and in cases where the secondary X is also employed, as assumedin Fig. 2, in inductive relation to coil P',"as in Fig. 1, it will have connected in its circuit the usual. well-known appara- -maximum inductance fora given number of ftuims, i. e., the greatest wave-lengthrange forthe most compact coil; Also in any case ifldesired, thewinding X of Fig. 1 may be combined with the apparatus shown in Figs. 3 and 4 in the'samc manner as is winding as described herein, and the user is to have the option of so employing either or both of coils P and.X.

The invention embodied herein is more particularly adapted as the receiver employed .radio communication.

The. contact-on-coils method, following the earlierpractice ofthe art in tuning by inductance variation consists in shifting two contacts ina straight line along the outside. 'o;t.'ahelicalcoil, ofwire wound with its Successive turns closely adjacent and in mutual inductive relation on a non-magnetic. core of'the high-frequency coil type, 2'. 6., as if'the contacts operate on the winding of this 9 1 e l f s; sl pes stra h as along the top of the coil, Fig. l. The two movable crntacts were also connected to the two sides of the high-frequency resonant circuit to be tuned, and the movements of the contacts to or from each other caused the inclusion of variable amounts of inductance in the high-frequency circuit, and the eut-cr-sciwice parts of the coil, on either side of the contact, being dead-ended.

One reason for the long-continued commercial success of the contact-on-coil apparatus is the fact that it permitted the disposition in compact form of the great lengths of wire arranged in many turns, which are i'iecessary to be used in tuning to widely differing-wavefrequencies within the limits of the extremely high frequencies of the order of hundreds of thousands per second which are employed in the radio telegraph service, this wire being thereby employed in such form that all its parts were successively in mutually inductive relation, and the entire structure, including coil and movable contacts, being of compact and convenient form; all of which said advantages are retained in the coils of the drawings herein, as shown more particularly in Figs. 1, 3 and l. Another practical advantage of the contact-on-coil device was that it permitted comparatively small variations of induce tance, 2'. e., as small as a single turn of the coil, (as distinguished from methods consisting in the switching in and out of entire groups of coil turns or else substituting alternately coils of differing lengths of wire), this turn-by-turn adjustment bein practicable and approximating su ciently closely for all practical purposes to the close tuning obtained with the continuousvariation type. This advantage is also retained in. the present invention, as will be explained in connection with drawings. However, the contact-on-coil method lacked an advantage which existed in the method of switching in definite groups of coil-turns, because in the former it was not possible to determine by inspection what the oscillation eriod of the circuit might be for any given adjustment of movable contact on the coil and because a sufiiciently speedy adjustment could not be made as between two widely di iiering frcquencieswithin the limits of the high frequencies employed.

' Among the other Ob'ects of this invention is that of producing a practicable inductance tuner which shall possess the advantages of the gradual variation of that of the contact-on-coil types. and also the advantage of substantially instant determination of the amount of the inductance in circuit for any and all adjustments, as well as the ability to quickly effect large variations of inductance. Such knowledge and ability to operate quickly is of the greatest importance in practical working,

that is, when with the knowledge, acquired'by experience, of just the number of turns which puts the circuit in tune with waves arriving from a given distant and known radiator or sendmg station. In tuningin anticipation of receiving acall from-such distant station, or in v tuning to waves from such station after a call from it has been received on a broad, general or so-called standby tune, it is of the utmost importance that the receiving operator lose no time in establishing the very best conditions of reception. But even if he knows the wave-length of the distant sender, he must wait until he can determine sueh'best conditions by trial, unless he can instantly shift his tuner into a condition known to correspond with that of the sender. The ability to permit such instantaneous shifting is an important object of this invention.

A further object of this invention is to provide an inductance tuner which shall permit quicker action than that of the contacv on-coil type, in respect of a change of tune involving a variation of a very large numher of coil-turns, which in said prior art involves the sliding of a movable contact over possibly many hundreds of turns of. Wire- It cannot be sufficiently emphasized that it is of the utmost importance in radio prac tice to enable the receiving operator to adjust his apparatus inthe speedi'est possible manner.

A simple combination of the contact-on coil method with the group-switching method would consist of an arrangement of the movable contacts of the first-named method along apart of the coil, for small adjustments, and in addition, for the large adjustments, the provisionof switches of the second-named method on the coil itself. But another object of this invention is to combine the advantages of both said two methods in a structure which may possess the advantages of both and the disadvantages of neither. A beginning is made by taking the high-frequency coil of the contact-on-coil type, the same consisting of a large number of turns of wire wound on a non-magnetic core, compactly in a single coiland lying closely together, not only to obtain a compact form, but to secure the best electrical arrangement consisting of the greatest inductance for a given number of turns, 5. e., the greatest range in wave-length adjustment for a given number of turns, that is, for a. given length of coil or degree of compactness of coilstructurc, all as employed in the apparatus shown in the drawings hereof. Next, in combining the small variations of the one method with the large variations of the other method, the plan is generally, in accordance with this invention, to follow the contact-on-coil method consisting in operatthe turns being arranged eliminating the parasitic currents.

coil with closely adjacent turns, but short taps and such a coil seemed inconsistent with suflicient spaces between the taps to prevent harmful capacity eifects, particularly if the entire apparatus were to be kept within reasonable space limits and without too long taps; and, for electrical reasons, as well as from a desire for compactness, it was not permissible in the case of these high frequcncy circuits (as it would be in the general electrical art of low frequency alternatinp currents) to wind the turns of the coil su'liiciently far apart so that the taps from such separated turns would involve no harm ful parasitic currents. Thus the problem was reduced down to some other method of This specific objectiof the invention is accomplished in the apparatus disclosed herein, by expedients so employed as to substantially eliminate all possible danger of parasitic currents. One of the more important of these expedients consists in allowing for suflicient space between the taps by reducing the number of the taps in such way as not to reduce the small regulations of the tuner. Among various other minor expedients is that which consists in providing means for so lixing the taps in position that they cannot assume sucl'i'relative positions as shall produce harmful parasitic currents.

The invention is best shown in Figs. 3 and 4, since Figs. 1 and 2 are largely diagrammatic and Fig. 1 lacksa showing of all the important details o the invention.

The invention will be best understood with particular reference to Fig. 3, which shows the essential features of the connections from the coil-turns to the box-wiring, from the latter to the switch-contacts, and from the latter to the switches and thence to the high frequency circuit, such as A, G, of Fig. 2, in which the coil is connected. In Fig. 3 is shown the inside or back of a board of side D of a box containing the coil or coils all the apparatus, including the switches and connections to which the coil or coils of Fig. l are connected. On the front of the board or panel D in Fig. 4 are only the operative surfaces of the contacts of the switches and also the movable switch arms S, S.

The apparatus mounted on the back or inside of the panel board, shown in Fig. 3, is as follows, the same constituting a permanent part of the panel and being combined therewith prior to the mounting in place in the box, of the coils of Fig. 1. Two

-:groups of connections are provided, one to and from connectors (l -2", $4, and S 10 and another to connectors 0, and 10"90. The numerals correspond with the numerals on the taps from the coil, as noted in Figs. 1 and 2. The b connectors are intermediate between the coil-taps and the a connectors. 'ihe b connectors are connected with the a connectors by box-wiring or coil-tap-continuations M and N. The b connectors may be employed when it is found necessary or desirable by such means to support the taps which extend intermediate the coil and the c connectors, for the purpose of keeping such taps separate from each other. This feature may be employed usefully also for the purpose of facilitating assemblage, particularly in such a case as in Fig. 3 when the connectors 1090 are remote from the open bottom of the box and hence not readily accessible for the purpose of connecting taps from the coil directly to said connectors l090, the coil itself being in place close to the panel D in order to permit the shortest possible coil-taps, and in such position making the connectors 10-9O yet more difiicult of access. An important remaining feature of the arrangement on panel D is the connection between connectors 0 and 0 by way of lead M, connector 0 and 1 By means of this system of panel connections, both connectors 0 and 0 are connected to the same fixed point of the coil, so that the two sets of connectors ()"10 and l0"90 are made cooperative. The coil tap 0 of Fig. 1 is connected tothe connector O (Fig. 3), and it is from this fixed intermediate point of the coil that tuning operations are conducted in opposite directions along the coil, and in such way that the operations are continuous in the sense desirable for best tuning, z'. 0., that all portions of the coil in circuit at any one time are continuous between their extreme opposite ends, including the point 0 and all the wire adjacent to that point on each side of it, this being the arrangement most highly preferred and the one made possible to perfection by the general arrangement hereof. Thus the advantage of the prior art is retained, consisting in operating in both directions from the middle of the coil, as above explained, and in this respect following the practice employed with thc contact-on-coil type, save that here the intermediate point is one which is definitely and permanently fixed. But in the case of the present improvement on that type, wherein taps are taken from the coil to switch-contacts, there would be hundreds of taps if one were taken from each turn of the coil for small adjustments throughout the length of the coil, and this I believe would cause serious parasitic currents in any practicable coil. Inorder to feet are disclosed hereinafter.

suitablemethods of obtaining the same cl- Various other arrangements equivalent-to that of Figs. 3 and "l are shown in Figs. 5-S described hereinafter, but the operation of all is substantiall the same and may be de scribed generally in terms of that of Figs. 3 and 4. the switch-contacts being marked to correspond with the number of coil turns in circuit, as shown. The tuning operations may be conducted by the operatorwith the assurance thatthe effect in tuning the high frequency circuit isuniform throughout the ntirelength of the inductance. i. c.. that there are no spots of inaccurate working on the coil resulting from parasitic currents due to the capacity of the taps and leads from the coils to the switch contacts.- and with the assurance that there are no inaccuracics due to the omission from the resonant circuit of any portion of the coil intervening between the extreme ends of the total portions of the coil in-circuit at any given time. In fact, when the number of taps is reduced by the expedients above described.

there is so little deleterious action ofparasitic currents that it becomes of much less importance thanwas anticipated to physically separate the taps from each other. in an apparatus of a given size, and the results of the operation of this invention seem to show that the naturally expect-ed difiiculties of parasitic currents in connection with these circuits of extremely high oscillationfrequency was considerably exaggerated as an obstacle in the minds of the prior workers in the radio art.

In operation, let-it be assumed that the switches of Fig. 3 are set at some general or stand-by tune. such as the radio receiving operator has found to be suitable for receiving messages from difi'erent senders, composed of widely difterent wave-lengths, say from three hundred meters to six hundred meters, up to five thousand meters. corresponding to frequencies per second of one million, one-halt million, and one hundred and eighty-seven thousand respectively. This stand-by tune might, for example.

be that to which the switches of Fig. 4 are shown as adjusted. z. 6., one corresponding to a tuner inductance for thirty-nine turns; which might be a suitable inductance adjustment, depending on the antenna, etc., for a six hundred meter wave having a frequency of a half-million per second. (It is understood that the coil P of Fig. 1 may comprise many hundreds of turns, in order to be 'adaptedto tune accurately and closely to any particular wave-length and frequency within the very wide limits em ployed in practice.) The operator then rece ves a call intended for him and signed by a distant sending station known to him from experience as having its waves best received by him on a given adjustment ofthe switches S and S, say for example, one corresponding to a tuner inductance ot' seventy-three turns, and say of a thousand meter wave-length with airequency of three hundred thousand per second. All that the operator has to do, (and this without any danger of faulty adjustment due to parasitic currents from hundreds of tap-leads in the box from the coil to the switch contacts), is to adjust the lower switch (Fig; 4) to contact and the upper switch to contact 3. On the other hand. if the distant sender hasa wavelength unknown to the operator, he need not experiment by making contacts throughout each of the hundreds of turns throughout the length of the coil, but only with the large-adjustmnet (lower) switch S over a tow contacts. and then, for perfect tune,

with the small adjustment (upper) switch S over a few contacts, this entire. tuning by trial occupying only a few seconds be.- fore he is ready to signal for the distant. sender to proceed with its message. The

receivingoperator may make a record of this adjustment for this particular sender, so that the next time it calls he can instantly set his tuner at the proper adjustment without any trial whatsoever. The receiving opbest results possible with a perfectly eflia:

dent tuner such as this. Furthermore, this successful solution of the parasitic current problems carries with it an elimination of all the contact difficulties above described as accompanying contact-shifting action directly on the wire turns of the coil itself.

As is indicated in l igs. 1 and 2. the variation of the inductance of coil P in the circuit of the grounded antenna A, G, serves not only to tune the antenna to the period of the waves being received, but also to vary the coupling of the primary and secondary circuits. The coils P, X, constitute an oscillation-transformer, which, owing to the lack of a magnetic core, provides a loose coupling between the primary circuit or receiving antenna and the circuit of secondary X which contains the usual receiving apparatus, and this loose coupling in turn provides mutual independence of the two circuits in respect of selectivity, and permits atransfer of energy by resonance from the antenna to the secondary circuit. Thus the action of this tuner in varying the cou- I tem, as well as dnivarying the antennaperiod, is very advantageous inrespect of its uniformity oi action resulting from the, described improvements. i

In Fig. 5 is shown an arrangement like,

' that of Figs. 1 and 3, except that twole'ads L L are taken from nei 'hborin" arts 0:5,

y the coil to respective zerorcontacts of the two switches, said zcrorontacts being connected by lead L, tliusproyidinga common zero connection for the, tWO S\V1t(JhS,,

, whereby any desired number of, turn units' may be connected in circuit. v,In this ,"case,

as in Fig. 1, all the turns are in the same ig. 4, arrangements may. made so,that ,both switches may operate in the same d1-" rection.

In Fig. fi thejleft portion ofthe coil is clockwise and the right hand portion is anti-clockwise, so that if thecommon zero connection of the two switches were tapped from the same point .(z'. ,e., the point of contiguity of the two parts of the coil as in Fig. i), the current would be invopposite directions in accordance .witlrthejopposite direction of winding. However, to correct this, the common zero connection is made as follows: The'zero taps L", L are taken from the two right hand. endsof the coil-parts, and these taps .are connected by lead L whereby the current is in auniform direction throughout, as indicated by the arrows. Also the unit switch contacts will be from right to left, in ascending .order, as shown, the same direction of reading as that of the tens, and this without crossing the taps asdescribed in connection with Fig. 4. Thus this arrangement of. Fig. 6 is substantially as efiicientas that of Figs, 2 and 5, the slight inferiority in Fig; 6.

1 consisting in the fact that for an operation which includes less than the total number of unit turns. there will be idle turns between the right handend-of theitens coil-1 part and the unit switch contact then in service. But in Fig. 6, as in the previous. figures, the. effect of cutting in additional unit turns by switch S is to add to the ef-- fective inductance in circuit.-

In Fig. 7 all the turns are wound in the same (2'. 6., clockwise) direction, as .in Fig;

5. The 'zero taps, L", L, L? .areled; offas, in'Fig. 6 however, andthe. direction off succession of the switch contacts is the same as in F ig. 6, the current direction'being therefore opposite in thetwo coil portions, as indicated by the arrows, andconsequently the cited; of cutting in unit turns by S is to proportionately reduce the effective inductance, the common zero connection being efi'ective,,as in the other cases, in permitting both switches to cooperate in including any desired number of turn-units in the circuit.

In, F ig, 8 the portion of the winding on the left is vclockwise, while that on the right hand portion is anti-clockwise, whereby the current in ,the two is in opposite directions.

But the specific common zero connection is 51111118.! to that of Fig. 5, as shown at L,

L? so that the effect of cutting in unit turns s to reduce the eiiective inductance in circuitg as in'the same of Fig. 7;the'common .zero connection, as in Fig. 7 as well as in all the other figures, being to coiiperateto include any desired numberof turn-units in, the circuit to be tuned.

lgln anapparatus for radio communicatiojn, means for tuning to resonance by inductance variation, a high-frequency oscillfltlOXl-CiIfClllbOf a receiving station, which comprises a high-frequency inductance-coil consisting of many turns of wire arranged around a non-magnetic coreand closely adjacent ,toeach other successively to provide maximum self-induction and wave-lengthrange tor minimum distance between" the ends, of the coil'; in combina'tion'with a switch-panel arranged alongside said coil; switch connectors' mounted on the same side of said panel as said inductance coil; .two switches with their contacts, mounted on the other side of said panehthe arms of the switches being adapted for connection with the high freque'ncy circuit to: be brought to resonance, and the switch contacts'being connected with said switcn-con- .n'ectors, on the coil-side of the panel; intervening connectors mounted on the coil-side of said panel; leads connecting the switchconnectors With said intervening connectors, and leads or taps from'said intervening connectors to said inductance coil; the 'connections and taps from the switch-contacts of each switch t'othe coil being of limited number and thereby avoiding serious parasi-tic currents, by means of an arrangement 'consisting in making-the connections from the switch-contacts of one switch, 'only to points of the coil which are separated by relatinely" large. portions of the coil, over- 'a. port on "of the coil up to a definite intermediat'epoint, and in providing for smaller adjustments, by" making the cdnnetions from i the switch-contacts of the other switchtd points of the coil beyond-said intermediate .point which are separated by relatively small portions of the coil; and

means for connecting said fixed point of the coil intermediate the large and small subdivisions thereof, with both saidswitchsets, whereby they both coiiperate to cutranged around a non-magnetic core andclosely adjacent to each other successively to provide maximum self-induction and wavelength-range for minimum distance between the ends of the. coil; in combination with a switch-panel arranged alongside vsaid coil; switch-connectors mounted on the same side of said panel as said inductance coil; two switches with their contacts, mounted on the other side of said panel, the arms of the switches being adapted for connection with the high-frequency circuit to be brought to resonance, and the switchcontacts being connected with said switchconnectors on the coil-side of the panel;

and connections from said switch-connectors to the high-frequency inductance coil,said connections being limited in number and thereby avoiding serious parasitic currents, by means of an arrangement consisting in making the connections from the switch-contacts of one switch, only to points ofthe coil which are separated by relatively large portions of the coil, over a portion of the coil up to a definite intermediate point, and in providing for smaller adjustments by making the connections from the switch-contacts of the other switch to points of the coil beyond said intermediate point, which are separated by relatively small portions of the coil; and means for connecting said fixed point of the coil intermediate the large and small subdivisions thereof, with both said switch sets whereby they both coiiperate to cut into the high-frequency circuit to be tuned, large and small units of inductance on opposite sides of said fixed intermediate point of zero inductance in circuit.

GREENLEAF WHIT'IIER PICKARD. 

